As mentioned before, there is AppScale which will let you avoid the lock-in if you so desire. AppScale has been around for quite a while, and it has been sponsored by Google, but it is an independent company and an open source project. AppScale has been running customers application reliably for few years, with Datastore load of up to a quarter million of transactions (and this is an old white paper).
AppScale will allow you to move your application (unchanged) and still reap the benefits of the App Engine model, autoscaling and all. I still believe App Engine benefits trumps any other PaaS out there.
Disclaimer: I'm part of the AppScale team, so somewhat partial to this topic :) Thanks for the kind words on AppScale: I'm very proud to be the last resort after google.
You mentioned App Engine's flaws. I understood price, and the sandbox restrictions, but you seem to imply the API is lacking also. Could you elaborate a bit more on it? What do you think is missing or lacking? What would you like to have that's not there?
The API is actually Open Source, Apache 2.0 to be precise, that's how AppScale could get started implementing the App Engine model. So why do you think it's a vendor lock-in?
AppScale will allow you to move your application (unchanged) and still reap the benefits of the App Engine model, autoscaling and all. I still believe App Engine benefits trumps any other PaaS out there.